While the present application is applicable to any wireless communications technology, the main ideas are explained in the context of the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology.
Frame Structure and Reference Signals
3GPP LTE technology is a mobile broadband wireless communication technology in which transmissions from network nodes such as base stations, referred to as eNBs below, to Wireless Communication Devices (WCD) such as mobile stations, also referred to as user equipment (UE), are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. A basic unit of transmission in LTE is a Resource Block (RB) which in its most common configuration comprises twelve subcarriers and seven OFDM symbols, i.e. one slot. A unit of one subcarrier and one OFDM symbol is referred to as a Resource Element (RE). Thus, an RB comprises 84 REs. An LTE radio subframe comprises two slots in time and multiple resource blocks in frequency with a number of RBs determining the bandwidth of the system. Furthermore, two RBs in a subframe that are adjacent in time are denoted an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs.
In the time domain, LTE downlink transmissions over a radio channel are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (DL), i.e. a link carrying transmissions from the eNB to the WCD, subframe may be transmitted from multiple antennas and the signal may be received at a WCD that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antennas or antenna ports. In order to demodulate any transmissions on the downlink, a WCD relies on Reference Symbols or Signals (RSs) that are transmitted on the downlink. These RS s and their position in the time-frequency grid are known to the WCD and hence can be used to synchronize the WCD to the downlink signal and to determine channel estimates by measuring the effect of the radio channel on these RSs. In Rel-11 and prior releases of LTE, there are multiple types of RSs. A Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS) are used for cell search and coarse time and frequency synchronization. Common Reference Symbols (CRS) are used for channel estimation during demodulation of control and data messages in addition to synchronization. The CRSs occur once every subframe. The Channel State Information—Reference Symbols (CSI-RS) are also used for channel state feedback related to the use of transmission modes that enable WCD-specific antenna precoding. These transmission modes use WCD-specific Demodulation—Reference Symbols (DM-RS) at the time of transmission of data with the precoding at the eNB performed based on the feedback received from and measured by the WCD on the CSI-RSs.
The PSS and SSS define a cell ID of a cell. The SSS can take 168 different values representing different cell ID groups. The PSS can take three different values that determine the cell ID within a group. Thus there are a total of 504 cell IDs. The PSS are Zadoff-Chu sequences of length 63 which along with 5 zeros appended on each edge of the Zadoff-Chu sequences occupy the 73 subcarriers in the central 6 RBs. The SSS are two m-sequences of length 31 that occupy alternate REs and are appended with 5 zeros on each edge of the m-sequences and located in the central 6 RBs as is the case for the PSS. The PSS and SSS sequences occur in subframes 0 and 5. The PSS sequence is the same in both subframe 0 and 5 while the SSS sequences differ between the subframes. The SSS sequence transmitted in subframe 0 is referred to as SSS1 while the SSS sequence transmitted in subframe 5 is referred to as SSS2. The SSS2 swaps the two length-31 m-sequences transmitted as part of the sequence SSS1 in subframe 0.
Dense deployments of small cells, e.g., cells served by low power base stations, are attractive to increase system capacity. However, dense deployments typically have fewer WCDs connected to each cell and lower resource utilization with higher rates provided when the cells are used. Reference signal structures that are developed for regular deployments with existing systems such as 3GPP LTE may have too high a density so that there is a lot of unnecessary interference created when deployments become dense. Reference signals may be transmitted even when there is no data being sent to WCDs.
In order to tackle this problem of unnecessary interference, solutions to turn small cells “off” when they are not being used are being considered, where “off” does not mean powered down, but rather not transmitting any data or merely transmitting small amounts of information infrequently, and “on” means a cell that schedules data to a WCD either in general or in the specific occasion when the data is actually scheduled. By turning off the base station in this way interference in neighboring cells may be reduced and the throughput may be increased in the wireless communication network. However, to ensure that cells can be ready to deliver data to and receive data from WCDs with minimal delay, it is necessary for WCDs to make some essential measurements on cells even when the cells are in an off mode. In order to facilitate this, a set of reference signals that are sent with much lower density in time have been discussed in 3GPP. Such signals are referred to as discovery signals and procedures associated with them are referred to as discovery procedures.
Discovery Signals
In “small cell on/off” scenarios in which an eNB can be “off”, e.g., not transmitting any data, for long periods of time, a discovery signal might be needed in order to assist the WCD with the measurements. The discovery signal needs to support the properties required for enabling Radio Resource Management (RRM) measurements, Radio Link Management (RLM) related procedures and coarse time/frequency synchronization. In order to make the measurements possible, the eNB has to wake up periodically, i.e. transition from “off” to “on”, e.g. once every 80 ms, or 160 ms, etc, and send the discovery signal so that it can be used by the WCD for mobility related operations such as cell identification, RLM and RRM measurements. Hence, the discovery signals are transmitted periodically indicating that the eNB exists.
In order to receive data on the downlink, WCDs need to be able to recognize when the small cell is on and when it is off. The highest gains are achieved when the transition time between the cell being on and off is as fast as possible. A problem that needs to be solved is to enable a sufficiently fast transition time while allowing the WCD to reliably detect when cells are on and off.
Existing solutions for signaling whether a node is active or not rely on some form of a preamble, e.g. in WiFi, some form of signaling from another carrier, e.g., activation/deactivation of a Secondary Cell (SCell) in LTE, or some form of a longer time scale cell search and discovery mechanism.
The problem with the general cell search and discovery mechanisms is that they take time, typically many tens of milliseconds, which is too slow to be useful as a fast dynamic indication of node activity, or cell activity, with fast transition times between on and off states or modes. This results in a reduced performance of the wireless communication network.